1. Field of the Invention
The present invention relates to a mobile communication system, and in particular, to a device and method for performing handoff in a mobile communication system.
2. Description of the Related Art
In a mobile communication system, there are several handoff methods for maintaining a call when a terminal (or mobile station) moves from a present cell (or base station) area to an adjacent cell area. One is a soft handoff method and another is a hard handoff method. In the soft handoff, the terminal communicates by using a channel allocated from a handoff target base station to which the terminal is to be handed off as well as a channel allocated from the present base station which presently services the terminal, and then disconnects one of the channels, whose channel quality is lower than a threshold value. In the hard handoff, when call quality is lower than a threshold level, the terminal first disconnects the channel allocated from the present base station and then attempts to connect with the adjacent base station. Furthermore, there is another handoff method in which upon receipt of a handoff request message, the base station transmits data for an interval shorter than a predetermined interval by increasing a data rate based on an engagement made between the base station and the terminal, and the terminal then receives the data for the above short interval and searches for an adjacent base station for the remaining interval. This handoff method is illustrated in FIG. 1.
FIG. 1 shows a state where a terminal (i.e., mobile station) moves from a present base station BS#0 to an adjacent base station BS#l (i.e., handoff target base station BS#1 to which the terminal is to be handed off). In this situation, the terminal perceives that the strength of a signal received from the present base station BS#0 is getting lower, while the strength of a signal received from the adjacent base station BS#1 is getting higher. In the meantime, when the strength of the signal received from the base station BS#0 is lower than a threshold level, the terminal transmits a handoff request message or a detected signal strength value to the current base station BS#0, and the base station BS#0 then divides the transmission frame cycle into a transmission interval Ton and a non-transmission interval Toff to transmit the entire frame data for the transmission interval Ton. Then, the terminal receives the signal from the base station BS#0 for the transmission interval Ton and receives the signal received from the adjacent base station BS#1 for the non-transmission interval Toff, to thereby perform handoff.
The technical terminologies used in the specification will be defined herein below. The term "mode 0" (e.g., a base station transmission mode 0 or a terminal reception mode 0) refers to a mode in which the base station normally transmits the data for the entire frame cycle and the terminal correspondingly receives the transmitted data. The term "mode 1" (e.g., a base station transmission mode 1 or a terminal reception mode 1) refers to a mode in which the base station transmits the entire data for a predetermined interval of the frame cycle and the terminal receives the transmitted data for the corresponding interval (i.e., the above predetermined interval). The term "mode 2" (e.g., a base station transmission mode 2 or a terminal reception mode 2) refers to a mode in which the base station does not transmit the data for a predetermined interval of the frame cycle and the terminal searches for an adjacent base station for the predetermined interval. Here, a guard time required in switching from mode 1 to mode 2 is represented by "a", and a guard time required in switching from mode 2 to mode 1 or mode 0 is represented by "b". Moreover, in dividing the frame cycle, reference character D1 denotes a leading interval of the frame and reference character D2 denotes a following interval of the frame. Reference character Ton denotes a data transmission interval for which the base station transmits the data to the terminal, and reference character Toff denotes a data non-transmission interval for which the base station stops transmission of the data to the terminal. Further, the term "first frame" refers to a frame transmitted primarily from the base station to the terminal at the handoff request, and the term "second frame" refers to a frame transmitted from the base station to the terminal in succession to the first frame.
Now, reference will be made to a conventional handoff method with reference to FIGS. 2 and 3. FIG. 2 is a diagram illustrating the relationship between the frame cycles and the base station transmission powers (or the terminal reception powers) during handoff in a conventional mobile communication system. In FIG. 2, reference character T denotes one frame cycle, reference character "a" denotes a guard time required in switching from the base station transmission mode 1 (or terminal reception mode 1) to the base station transmission mode 2 (or terminal reception mode 2, reference character "b" denotes a guard time required in switching from the base station transmission mode 2 (or terminal reception mode 2) to the base station transmission mode 1 (or terminal reception mode 1), and reference character "S" (i.e., S/2+S/2) denotes an actual search time for which the terminal searches for the adjacent base station over two frame cycles 2T. In FIG. 2, the time S can be expressed by S=2.times.(Toff-a-b)=2Toff-2a-2b. As illustrated, in the conventional handoff method, the base station transmits the data for the transmission interval Ton=T/2 out of one frame cycle and stops transmission of the data for the non-transmission interval Toff=T/2.
When the base station transmits the data as shown in FIG. 2 in a handoff mode, the terminal performs handoff according to the procedure shown in FIG. 3.
Referring to FIG. 3, the terminal receives and processes the frame data transmitted normally from the base station (step 310). In this state, the terminal transmits a handoff request message to the base station, if it is determined that handoff is required, for example, due to the reduced strength of the signal received from the base station. The base station then informs the terminal of the handoff, successively transmits the data at a double data rate for the leading half cycle T/2 of the frame cycle T, and stops transmitting the data for the following half cycle T/2. Then, the terminal senses, in step 312, that the handoff is initiated, and receives the data of double data rate for the leading half cycle T/2 of the frame cycle T (step 314). The relation between the leading half cycle T/2, for which the terminal receives the data, and the base station transmission power is represented by reference numeral 200 in FIG. 2. Thereafter, the terminal searches for the adjacent base station, to which the terminal is to be handed off, for the following half cycle T/2 (step 316). Actually, however, the terminal does not search for the adjacent base station for the whole remaining half cycle T/2, but for the time obtained by subtracting a guard time "a" and a guard time "b" from the following half cycle T/2, wherein the guard time "a" is a time required in switching from the base state transmission mode 1 (or terminal reception mode 1) to the base station transmission mode 2 (or the terminal reception mode 2) and the guard time "b" is a time required in switching from the base station transmission mode 2 (or terminal reception mode 2) to the base station transmission mode 1 (or terminal reception mode 1). Accordingly, the actual time for which the base station searches for the adjacent base station is S/2=T/2-a-b. The relation between the actual search cycle and the base station transmission power is represented by reference numeral 210 in FIG. 2.
In the meantime, the cellular terminal determined whether the adjacent base station is detected (i.e., whether a signal is detected from the adjacent base station) (step 318). When the signal is detected from the adjacent base station, the cellular terminal is handed off to the adjacent base station in step 320. After completion of the handoff, the terminal returns to step 310 and continues to normally receive the data. However, upon failure to detect the signal transmitted from the adjacent base station in step 318, the terminal returns to step 314 to receive the data for the following half cycle T/2 of the next frame cycle. Here, the relation between the respective half cycles for the next frame and the transmission powers are represented by reference numerals 220 and 230 in FIG. 2.
As stated above, although a theoretical time available for the adjacent base station search is 2Toff, the actual time S available for the adjacent base station search is S=2Toff-2a-2b. That is to say, as can be appreciated from FIG. 2, the reception mode 2 follows the reception mode 1 with a time delay "a", and the reception mode 1 or 0 also follows the reception mode 2 with a time delay "b", thereby reducing the actual search time by the guard time required in switching the modes.
That is, in the conventional handoff method, operations of transmitting the data and searching for the adjacent base station are repeated until the adjacent base station to which the terminal is to be handed off is detected, which wastes the guard time required in switching the transmission/reception modes. The waste of the guard time will reduce the actual search time for which the terminal searches for the adjacent base station. Furthermore, the frequency mode switching overloads both the base station and the terminal.